1. Field of the Invention
The present invention relates to the production of superabsorbent acrylic polymer powders capable of absorbing large amounts of water or aqueous fluids and which are well suited for a variety of hygienic applications.
2. Description of the Prior Art
It is known to this art to produce polymer particulates having a high capacity for water absorption by inverse suspension polymerization of ethylenically unsaturated monomers, more particularly of acrylic monomers. The powders which are thus obtained swell greatly in the presence of water, providing gels of high mechanical strength. These properties are useful, inter alia, for the manufacture of sanitary appliances, e.g., sanitary napkins, for absorption and retention of body fluids.
Such polymers, however, contain not insignificant amounts of residual monomer. This presents a serious problem because these monomers are toxic or at least irritating or damaging to mucous membranes and human skin. Various more or less satisfactory solutions have been proposed to this art for reducing the residual monomer content of the hydrophilic acrylic monomers, for example treatment with isopropanol (EP-0,262,380, Cassella), with hydroxylamine (U.S. Pat. No. 4,929,717, Stockhausen), with an amino acid such as lysine (U.S. Pat. No. 4,766,173, Nalco) or with azo compounds or peroxides (JP-89/103,644, Sanyo). The latter, in particular, relates that the monomer content can be reduced to a level of 500 ppm either by introducing an oxidation-reduction couple into the mass undergoing polymerization, or by treating the polymer formed, but prior to its final drying, with 0.01 to 100 ppm of hydrogen peroxide relative to the polymer (expressed as dry weight), 0.01 ppm being a lower limit below which no significant effect on the residual monomer content is observed, and 100 ppm being an upper limit which cannot be exceeded without adversely affecting the absorption capacity of the final product.
One significant improvement in the production of such absorbent powders is described in EP-0,441,507, comprising polymerization of the acrylic monomer in at least two separate stages. In a first discrete stage, an inverse suspension polymerization is carried out in conventional manner, such polymerization resulting in the formation of a gel. In a second stage, a fresh monomer charge is absorbed into this gel and polymerization thereof is initiated within the actual gel formed previously. If appropriate, this absorption/polymerization sequence can be repeated. In this fashion, polymeric resins are prepared having a particle size which is appreciably larger than the resins obtained via single inverse suspension polymerization. Their degree or extent of swelling in the presence of water, elastic modulus, plasticity and resistance to collapse under pressure of the gel, are also appreciably improved.